The invention relates to a process for the partial combustion of finely divided solid fuel and a burner for use in such a process.
Partial combustion--also referred to as gasification--of solid fuel can be achieved by reaction of the solid fuel with oxygen. The fuel contains as useful components mainly carbon and hydrogen, which react with the oxygen--and possibly with steam and carbon dioxide--to form carbon monoxide and hydrogen. Depending on the temperature, the formation of methane is also possible. While the invention is described primarily with reference to pulverized coal the process and burner according to the invention are also suitable for other finely divided solid fuels which can be partially combusted, such as for example lignite, pulverized wood, bitumen, soot and petroleum coke. In the gasificiation process pure oxygen or an oxygen containing gas, such as air or a mixture of air and oxygen, can be used. All of the above are referred to as oxygen.
In a well known process for partial combustion of solid fuel, finely divided solid fuel is passed into a reactor at a relatively high velocity. In the reactor a flame is maintained in which the fuel reacts with oxygen at temperatures above 1OOO.degree. C. Since the residence time of the fuel in the reactor is relatively short, the risk of sintering of the solid fuel, which might cause plugging, is minimized. This aspect makes the above process suitable for the gasification of a wide range of solid fuels, even solid fuels having a tendency to sinter. The solid fuel is normally passed in a carrier gas to the reactor via a burner, while oxygen is simultaneously introduced into the reactor via said burner. Since solid fuel, even when it is finely divided, is usually less reactive than atomized liquid fuel or gaseous fuel, great care must be taken in the manner in which the fuel is dispersed in and mixed with the oxygen. If the mixing is insufficient, zones of underheating are generated in the reactor, next to zones of overheating, caused by the fact that part of the solid fuel does not receive sufficient oxygen and another part of the fuel receives too much oxygen. In zones of underheating the fuel is not completely gasified, while in zones of overheating the fuel is completely converted into less valuable products, i.e. carbon dioxide and water vapor. Local high temperatures in the reactor have a further drawback in that these will easily cause damage to the refractory lining which is normally arranged at the inner surface of the reactor wall.
In order to ensure a good mixing of fuel and oxygen it has already been proposed to mix the fuel and oxygen in or upstream of the burner prior to introducing the fuel into the reactor space. This implies, however, a disadvantage in that--especially at high pressure gasification--the design and operation of the burner is highly critical. The reason therefore is that the time elapsing between the moment of mixing and the moment the mixture enters the reactor must be invariably shorter than the combustion induction time of the mixture. The combustion induction time, however, considerably decreases with a rise in gasification pressure. When supplying a small quantity of fuel together with a small quantity of oxygen or oxygen-containing gas, the total velocity of the mixture in the burner will be low, so that the combustion induction time may be easily reached in the burner itself, with the risk of severe damage to the burner construction. The above problem of the risk of premature combustion in the burner could be avoided by mixing the fuel and oxygen outside the burner in the reactor space. In this case special provisions should be taken to ensure a good mixing of fuel and oxygen, necessary for a proper gasification. A drawback of mixing fuel and oxygen in the reactor outside the burner is, however, the risk of overheating of the burnerfront, due to a hot flame front caused by premature contact of free oxygen with already formed carbon monoxide and hydrogen in the reactor.